Some devices comprise a hollow drive shaft through which a rod-shaped object is passed. While the drive shaft is rotating the rod-shaped object remains fix.
An example for a device in which this kind of arrangement is used is a spin unit in wafer production. More specifically, a cleaning unit in wafer production comprises an electrical motor, a so-called spin motor, comprising a hollow shaft. The motor shaft may rotate with a velocity of about 1000 to 5000 rounds per minute (rpm). A common velocity is about 3000 rpm. The shaft of the spin motor is attached to a wafer holder for turning or spinning a wafer quickly.
Inside the hollow shaft a rinsing nozzle is mounted. Through the rinsing nozzle de-ionized water (DIW) is led to the wafer for high pressure rinsing or cleaning.
The nozzle is mounted to be stationary while the shaft turns round.
The drive shaft as well as the rinse nozzle is made of metal. The nozzle is mounted so that it is spaced from the inside wall of the drive shaft.
In practice, the metal nozzle must be remounted into the shaft after repair, preventive maintenance or still other occasions.
If due to an imperfect mounting the nozzle contacts the inside wall of the drive shaft this is not visible because it is inside the shaft. A slight contact will not hinder movement of the drive shaft either. However, grinding of the nozzle against the drive shaft leads to an abrasion of metal particles. These metal particles which may be very small are transported to the wafer surface due to the under-pressure in the cup in which the wafer is mounted.
As a result, short circuits occur on the circuits realized on the wafer due to the metallic particles. As entire wafers are concerned the influence on the production is important, the scrap rate increases severely. Furthermore, it is very difficult to locate the error source.